Capacitors are commonly used in a variety of electrical applications. For example, capacitors are used to tune the frequency of radio and television receivers, to eliminate sparking in automobile ignition systems, as energy storing devices, in electronic flashing units, and as filters in power supplies such as variable speed, constant frequency power generation systems used for providing three phase electrical power. In a variable speed, constant frequency power generation system, a variable speed mechanical input, such as the engine of an aircraft, mechanically drives a generator at a variable speed. Because the generator is being driven at a variable speed, the frequency of its output signal is consequently variable. A variable speed, constant frequency power generation system converts the variable frequency output from a variable speed, mechanically driven generator into constant frequency alternating current for use by alternating current loads.
In a typical variable speed, constant frequency power generation system, a rectifier converts the variable frequency output from a variable speed, mechanically driven generator into a DC signal. An inverter then inverts the DC signal from the rectifier into constant frequency AC output power. Capacitors are typically used both at the output of the rectifier for smoothing the DC signal provided by the rectifier and at the output of the inverter for eliminating unwanted harmonics of the inverter fundamental frequency from the constant frequency AC output power.
Capacitors generally consist of two or more electrodes separated by a dielectric such as air or other material having a desired permittivity. The amount of capacitance of a capacitor is dependent upon the surface area of the electrodes of the capacitor, the distance separating the electrodes, and the permittivity of the dielectric separating the electrodes.
A capacitor can have a variety of geometric constructions. A parallel plate capacitor, for example, is a capacitor in which the electrodes thereof are parallel plates separated by a dielectric having both a thickness and a permittivity selected to control the amount of capacitance of the capacitor.
A cylindrical capacitor is a capacitor in which one of its electrodes is a first cylindrical hollow tube and another of its electrodes is a second cylindrical hollow tube concentric with the first cylindrical hollow tube. A dielectric between the first and second cylindrical hollow tubes has a thickness and a permittivity selected to control the capacitance of the cylindrical capacitor. A spherical capacitor has one electrode in the form of a hollow sphere surrounding another electrode in the form of a usually solid concentric sphere. The volume between the hollow sphere and the concentric sphere contains a dielectric having a thickness and a permittivity selected to control the capacitance of the spherical capacitor. A cylindrical film capacitor typically consists of a sandwich construction having at least four strips. These four strips are, in order, a first conducting strip, a first dielectric strip separating the first conducting strip from a second conducting strip, and a second dielectric strip. The four strips are wound such that, if the wound capacitor is viewed on end, the layers of the capacitor have a spiral appearance. In the resulting capacitor, the first and second dielectric strips electrically insulate the first and second conducting strips from one another.
One form of a parallel plate capacitor is a multiple layer capacitor (MLC) having two electrodes wherein each electrode comprises a plurality of parallel plates. The plurality of plates of the first electrode are electrically interconnected at one end thereof, and the plurality of plates of the second electrode are electrically interconnected at one end thereof. The plates of the two electrodes of this capacitor are interleaved so that, except for one outermost plate of each electrode, each plate of each electrode is sandwiched between two plates of the other electrode. A dielectric electrically insulates the plates of one electrode from the plates of the other electrode.
Various arrangements for electrically supporting a capacitor, such as a multiple layer capacitor, have been provided in the prior art. One common arrangement for electrically supporting a capacitor is a printed circuit board which has receptacles for receiving pins of the capacitor. The pins are inserted through holes in the printed circuit board and are then soldered to appropriate circuit paths on the printed circuit board. Thus, the solder used in soldering the capacitor to these circuit paths not only electrically connects the capacitor to the printed circuit board but also facilitates the support of the capacitor by the printed circuit board. It is often advantageous in many applications, such as in a variable speed, constant frequency power generation system, to use a plurality of smaller capacitors, each of which may be similar to the aforementioned multiple layer capacitor, in place of one large capacitor. Since the total capacitance of a plurality of parallel connected capacitors is equal to the sum of the individual capacitances of the capacitors, it is necessary to electrically connect these smaller capacitors in parallel in order to realize the same capacitance as the larger capacitor. Accordingly, it is often necessary to mount a group of capacitors (for example, as many as eight) together so that the capacitors are electrically connected in parallel.
One way of mounting a plurality of capacitors is to solder them together and to a frame. However, this arrangement can induce large mechanical stresses in the electrodes of the capacitor and in the dielectric which separates those electrodes. These mechanical stresses arise due to the presence of large temperature changes, such as those resulting when the capacitor cools down from its soldering temperature to the operating temperature of its environment. Mechanical stresses may also arise due to clamps used in mounting the capacitors. Accordingly, it is desirable to mount a plurality of capacitors so as to avoid such mechanical stresses and so that the capacitors are electrically connected in parallel. It is also desirable to mount these capacitors in such a way as to ensure that current is evenly distributed between the capacitors.